Flexible, slabstock polyurethane foam is an open-celled, porous foam that is widely used in furniture, mattresses, carpet underlay, and many other applications for which cushioning is desired. The polymerization reaction to make polyurethane foam is exothermic, and much of the heat generated in the reaction is trapped within the foam, which is a good insulator. Foams are typically left to cool and cure for hours, or even days, to allow them to stabilize. Unfortunately, even foams that are allowed to cure slowly at ambient temperature do not usually develop uniform physical properties throughout the foam, largely because of the vast temperature gradient that develops across the foam during and after polymerization.
To overcome some of the problems of uneven curing of polyurethane foams, Ricciardi et al. (U.S. Pat. No. 3,890,414) developed a rapid-cool process in which a freshly polymerized polyurethane foam bun is uniformly cooled by passing a large quantity of a cooling gas through the foam mass, preferably by applying a vacuum to one surface of the foam bun. This process is also known as "forced-air cooling."
The rapid-cool process described in U.S. Pat. No. 3,890,414 reduces the amount of time required for cooling foams, and gives products with more uniform physical properties. A drawback of the rapid-cool process is that the cooling gas initially exhausted from the foam contains particulate matter as is evidenced by a visible smoke plume. When large foam buns are produced, a substantial amount of smoke is exhausted to the atmosphere, and this smoke is a concern to slab foam producers.
Recent patents describe newer approaches to post-treating freshly made flexible polyurethane foams. U.S. Pat. No. 5,188,792, for example, teaches to post-treat a polyurethane foam by recirculating reactant-laden air through a foam block that has been slit along opposing faces.
U.S. Pat. No. 5,171,756 teaches a three-stage cooling process. In stage one, cooled air is drawn through the foam and is then exhausted to the atmosphere. The reference suggests that the exhausted air contains excess water, BHT, and a minor proportion of TDI ureas. This air is exhausted to the atmosphere to prevent BHT and TDI ureas from clogging heat exchangers. In stage two, sublimates are withdrawn from the foam with air that is subsequently cooled to condense the sublimates, and is then recirculated through the foam to redeposit the sublimates uniformly throughout the foam. In stage three, additional cool air is drawn through the bun and is vented to the atmosphere to remove moisture and volatile components.
Unfortunately, when cooling air is exhausted to the atmosphere during stage one of the process of U.S. Pat. No. 5,171,756, a visible smoke plume is still evident. Thus, although rapid-cool technology that permits more uniform curing and cooling of polyurethane foams is available, the problem of eliminating visible smoke generated during at least the initial stage of the rapid-cool process remains.